Cathodic arc deposition of metal-rich Cr-based coatings

Abstract: Wood and wood-based products have always been key components in the development of human civilization. Nowadays most wood harvesting is done using chainsaws, and their efficient operation helps promote worker safety and reduce emissions. An important factor for efficient operation is the sharpness of the chain – a sharper chain requires less energy for cutting and reduces the risk of dangerous kickback. To reduce the rate of wear and prolong the life of the chains, they are protected by a hard chrome coating. The production of this coating involves compounds in which Cr atoms are in the toxic and carcinogenic hexavalent state. The deposited coatings contain no toxic compounds, however. Consequently, there is a need to develop replacement processes which can produce equally well-performing coatings.In this thesis I investigate the process of cathodic arc deposition of coatings for this purpose. Cathodic arc deposition is a well-established technique for tool coatings, free of toxic compounds. Specifically, elemental Cr coatings, and Cr-rich Cr-N, Cr-C, and Cr-C-N coatings are studied.For the study on elemental Cr, focus is put on the impact of substrate bias on the growth and coating properties. This is important in cathodic arc deposition as the evaporated species are ionized to a high degree. I show that an increasing substrate bias increases the temperature of the substrate and compressive stress of the coatings while decreasing the growth rate because of resputtering. The texture also changes from a preferential [110]- to [100]-orientation, and the hardness is lower than for typical hard chrome coatings.For alloying with N or C, using N2 and C2H2, respectively, the N-/Ccontent is shown to decrease with increasing bias. This process is attributed to preferential resputtering, and ceases at a critical partial pressure, different for N and C, at which point there is no observed difference in N- or C- content at different bias. The incorporation strongly affects the microstructure of the coatings. For N, there is a transition from a columnar structure to a featureless appearance at high N-content, while for C a mixed amorphous/crystalline structure appears at high C-content. For both C and N, the change in microstructure is accompanied by a large increase in hardness, which almost doubles when the alloying concentration reaches ~7 at.%.In a fourth study, combined C- and N-alloying is investigated. Here it is seen that the preferential resputtering persists for N, but for C there is a linear increase in content with partial pressure of C2H2 for all bias levels. This results in the coatings containing more C than N, with a structure like that of Cr-C. At the highest alloying level, a new structure appears which is not seen for the Cr-N and Cr-C studies, consisting of small, elongated grains and columns.In a fifth study, synchrotron-based X-ray adsorption spectroscopy is used to provide structural information about the coatings that is difficult to access with X-ray diffraction. The measurements show that Cr2N and Cr3C2 are the predominant phases forming alongside bcc Cr and reveal changes in bond orientations and strengths at differing alloying concentrations.